The table below shows the color of three indicators at different pH values:

Question

Indicator | pH=0 | pH=2 | pH=4 | pH=6 | pH=8 | pH=10 | pH=12 | pH=14
Litmus paper | Red | Red | Red | Red | Blue | Blue | Blue | Blue
Phenolphthalein | No change | No change | No change | No change | No change | Pink | Pink | Pink
Thymol blue | Red | Red | Yellow | Yellow | Yellow | Blue | Blue | Blue

An unknown liquid is tested with three indicators:
• The unknown liquid turns litmus paper blue.
• The unknown liquid is not affected by phenolphthalein indicator.
• The unknown liquid turns yellow when tested with thymol blue indicator.

What is the pH of the unknown solution?

Answer 1

To determine the pH of the unknown solution based on the information provided from the indicators:

1. Litmus Paper: The liquid turns litmus paper blue, indicating that the pH is above 8 (as it is red below this pH and blue at or above 8).

2. Phenolphthalein: The liquid is not affected by phenolphthalein. This means it does not show a color change, indicating that the pH is below 10, as phenolphthalein changes to pink at a pH of 10.

3. Thymol Blue: The liquid turns yellow when tested with thymol blue. Thymol blue is red at low pH values (pH < 1.2), yellow at intermediate pH values (between 6 and 8), and blue at high pH values (above 8). Since the unknown liquid turns yellow, it indicates that the pH is likely around 6 to 8.

From these points, we can deduce the following:

• The pH is greater than 8 (from the litmus paper).
• The pH is less than 10 (from phenolphthalein).
• The pH is around 6 to 8 but specifically yellow points to a shift towards 8.

Thus, the only possible pH that satisfies all these conditions is pH 8.

Therefore, the pH of the unknown solution is 8.